The present invention relates to a method and apparatus for reading optically encoded symbols, and is directed more particularly to a method and apparatus for reading optically encoded symbols that uses multiple optical scanning assemblies having differing spatial resolutions and at least partially overlapping depths of field.
Optical readers, particularly bar code readers, are being used in an ever widening range of applications. In those applications where the amount of data is relatively small, the data is conveniently encoded in a one-dimensional (1D) bar code symbol. The reading of data from such symbols requires an optical reader with a spatial resolution that is relatively high in only a single dimension.
For 1D symbols which are relatively large or have relatively low data densities, readers which use 1D illumination sources and photosensitive detectors such as 1D or linear image sensors, are often used. Example of such 1D readers are described U.S. Pat. Nos. 5,187,355 (Chandler) and 5,288,984 (Ito et al). For 1D symbols which are relatively small or have relatively high data densities, on the other hand, readers which use scanning lasers and photosensitive detectors such as photodiodes are often used. Examples of such readers are described in U.S. Pat. Nos. 4,387,297 (Swartz et al) and 4,409,410 (Shepard et al).
In those applications where the amount of data is relatively large, the data is preferably encoded in a two-dimensional (2D) bar code symbol. The reading of data from such symbols requires an optical reader which has approximately equal spatial resolutions in two mutually perpendicular directions.
For 2D symbols which are relatively large or have relatively low data densities, readers which use 2D or flood illumination sources with photosensitive detectors such as 2D or matrix image sensors are often used. Examples of such 2D readers are described in U.S. Pat. Nos. 4,488,679 (Bockholt) and 5,531,366 (Wang et al). For 2D symbols which are relatively small or have relatively high data densities, on the other hand, readers which use rasterized scanning lasers with suitable photosensitive detectors are often used. Examples of such 2D readers are described in U.S. Pat. No. 5,545,886 and in International Patent Publication no. WO93/18478 (Durbin et al).
The problem with readers of the above-mentioned types is that, due to the ever widening range of products to which optically readable symbols are being applied, users frequently encounter situations in which their readers are unable to successfully decode symbols of a variety of types, sizes and densities. Significantly, this inability is more often the result of a mismatch between the size or density of the symbol and the resolution and depth of field capability of the reader, than it is the result of an inability of the reader to autodiscriminate and decode symbols of different types.
In view of the foregoing, it will be seen that a need exists for an optical reader that is able to read and decode bar code symbols having a wide range of resolution requirements, and for a method of operating such reader that eliminates the need to manually reconfigure the reader as symbols having different sizes and densities are encountered during use.